The inventive concept relates generally to nonvolatile memory devices. More particularly, the inventive concept relates to nonvolatile memory devices comprising multi-level cells, and related methods of programming.
Flash memory devices in the past were limited to storing only one bit of data per memory cell. Recent technological advances, however, have allowed researchers to develop flash memory devices storing more than one bit of data per memory cell. Flash memory devices storing one bit of data per memory cell are referred to as single level cell (SLC) devices, and flash memory devices storing more than one bit of data per memory cell are referred to as multi-level cell (MLC) devices.
A flash memory device can store “N” bits of data per memory cell by programming each memory cell to one of 2N different threshold voltage distributions. For instance, a flash memory cell can store two bits of data per memory cell by programming each memory cell to one of four (22=4) different threshold voltage distributions corresponding to four different two-bit states “11”, “10”, “00”, and “01”. The memory cells in the 2N different threshold voltage distributions can be read by using different read voltages to determine the threshold voltage required to turn on each memory cell. A read voltage that is higher than a memory cell's threshold voltage will turn on memory cell on, providing an indication of the threshold voltage distribution to which the memory cell is programmed.
To ensure accurate read operations, the 2N threshold voltage distributions must be separated from each other by an adequate read margin. If two threshold voltage distributions overlap each other or are too close together, memory cells belonging to one distribution may be erroneously read as belonging to the other distribution. Unfortunately, the increasing performance demands of modern flash memory devices continue to require smaller read margins. Moreover, certain aspects of newer flash memory devices, such as increasing integration density, may cause threshold voltage distributions to widen due to electrical coupling between adjacent memory cells.